杯(はい, ぱい or ばい)
It is used when you count cups, glasses and bowls.

がんばる子
杯 is used when you have food or drink in it.
Use 個(こ/ko)if you don’t.

When you count, put whatever you count first, then the counter.
Example
- three cups of coffee: コーヒー二杯
- four glasses of beer: ビール三杯
- two bowls of ramen : ラーメン二杯